Signaling



' ducing distortion.

Patented Oct. 5, 1937 UNITED STATES SIGNALING Harold H. Beverage, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 26, 1933, Serial No. 668,098

8 Claims.

This invention discloses a method for transmitting and receiving over a single sideband and carrier, double sideband and carrier or single sideband only. It may also be applied to either amplitude, phase or frequency modulation.

Briefly, the idea is to modulate a relatively low frequency carrier, select a single sideband by means of a band-pass filter and combine this single sideband with a second carrier in such a manner that there is a space between the second carrier and the group of sidebands which carry the intelligence. Due to this spacing between the carrier and the sidebands, it is relatively easy to select the carrier and a single sideband or a single sideband only, in the further stages containing conventional modulators. Furthermore, if the energy is radiated With an idle space between the carrier frequency and the intelligence bearing sidebands, it is relatively easy to design a receiver which can select the sidebands independently of the carrier.

In the prior art, it has been customary to modulate a carrier in such a way that sidebands extend outward from the carrier with no idle space between the carrier and the sidebands representing the lower modulation frequencies. This method of signaling has several disadvantages. In the first place, during transmission through the ether, particularly at short wavelengths, the fading is not uniform over the band of frequencies transmitted. This phenomena, which is known by the term selective fading, results in impaired quality. For example, if the carrier frequency should fade out leaving the sidebands, the two sidebands beat together producing a great many harmonic frequencies which did not exist at the transmitting station. If the carrier does not fade completely out, the effect is not as serious, but, nevertheless, distortion is produced since the beating between the sidebands and the reduced carrier is equivalent to overmodulation. Furthermore, the phase relations be tween essential frequencies in the upper and lower sidebands, are continually changing, also pro- In order to eliminate these distortions, it is highly desirable to eliminate one of the sidebands, or at least to receive each sideband separately, and combine them afterwards in their proper phase relations.

In the ordinary method of modulating a transmitter, the sidebands are so close to the carrier that it is difficult to separate the sidebands from the carrier and from each other. In the system which I am proposing, the sidebands and the carrier are separated in frequency a sufficient amount to make it possible to select them separately at the receiver.

The novel features of my invention have been set forth with particularity in the claims appended hereto as required by law.

The art of signaling in accordance with my novel method and the .means necessary to carry out such signaling will be best understood by reading the detailed description which follows and therefrom when read in connection with the attached drawings, in which:

Figure 1 illustrates diagrammatically one means for transmitting signals developed in accordance with my invention;

Figure 2 shows a receiver of signals developed in accordance with my method;

Figure 3 shows a modification of Figure 2; while Figure 4 illustrates a correction circuit which may be used at the transmitter or the receiver to alter the character of the same whereby phase modulation and demodulation may be accomplished.

In Figure 1 I have shown the setup for one form of transmitter embodying my invention. I have divided the various operations into ten sepa' rate stages and below the block diagram repre-" senting the various stages I have shown the frequencies which pass through each portion of the equipment. This is merely a typical case to aid in the description of the operation of my invention.

In stage I a microphone M, adapted to be energized by speech waves, is connected with a speech amplifier as shown. The frequency range of the microphone and amplifier is assumed to be zero to 5 kilocycles, as shown by the block diagram directly below stage I, although, of course, in actual practice, it might be from 30 cycles to 5,000 cycles. Stage 2 includes a modulator ll in which a carrier frequency of kilocycles, supplied from a carrier source as shown, is modulated by the speech frequencies, producing two sidebands extending from 15 to kilocycles, as indicated, by the energy diagram directly below stage 2. A band pass filter in stage 3 selects the lower sideband of 15 to 20 kilocycles, as shown diagrammatically below stage 3. This lower sideband is then combined in the modulator of stage 4 with the 25 kilocycle carrier from any source in a second modulator 20. The lower sideband extends from 5 to 10 kilocycles and the upper sideband extends from to kilocycles as shown diagrammatically in the energy diagram below stage Between the carrier of 25 kilocycles and each side-- band there is a blank space of 15 kilocycles. The lower sideband may be selected by a low pass have an idle zone containing no frequencies be-- filter included in stage 5 which filter allows all frequencies below'10 kilocycles to pass. -We now tween zero and 5 kilocyolesand the speech intelligence bearing frequencies are concentrated between 5 and iookilocycles; as shown below stage 5; This band of frequencies is applied to stage 6 V "which includes a third modulator 30, whereit modulates a carrier of 20 kilocycles suppliedfro m a. source as shown; From this modulation an upper sideband between 205 and 210 kilocycles; and a lower sideband-between 190 and 195 kilo cycles are obtainedgas shown by the graphs be-, The uppersideband and carrier low stage 6. are'selected: by ,a band-pass filter included in stage 1, whichpasses 200 to 2l0 kilocyclesl Theoutput of the filter instage 8 modulates a 10,000

1 kilocycle carrier'which' includes a; fourth modu- 'lator 40. A band-pass filter passing 10,200kilocycles'to 110,210 kilocycles included in stage-9g selects the uppersideband' only, which, it' will be noted by inspection of the .graph below stage'll,

consists of a carrier, a kilocycle space and a single 5' kilocycle sideband; Modulator ma be arrangedto suppress the;10,000 kilocycle carrier and it will be noted that. the various side- 7 bands are well spreadfout,making it possible' to QaerialA. I I V V 7 Th generators, filters and'modulators used in described in detail here; However, thelgenerators' 'select'the desired frequencies with a relatively inexpensive filter. The 'output of the final filter is amplified by a power amplifierfincluded in stag l0 andfradiated into space from .an

this systemlper se form'no part ofthe present invention and'for ith at reasonhave not been should 'provide oscillations of substantially, constant frequency and "amplitude. The moan-1; 'lators may be of any known type and may modu? late the oscillationsinhny of its characteristics including phztsafrequen'cy and amplitude. 'Likewise, the filters may be of any known type which a have the desired cutoff characteristics, etc. For purposes ofillustration' I have assumed that the oscillations are to be modulated in, amplitude:

Figurez shows airecelver which may lbe'used' lite receive the radiationsifrom the transmitter ofFigurel. 'I'he signals'including the carrier and 'fthe' sideband from the carrier, as shown above the'filter. It will be 1 notcd thatfthej carrier is removed 5 kilocy'clestfrom the lowest frequency in;the sideband,corresponding to a separatioir r [of 5Jpercent.,'which makes it possible toseparate' Ithefsideband frojn' theicarrier with'fa relatively;

7 inexpensive band-pass'filterQZA. I

;In i the intermediategfrequency detector 7 sideband frequencies are combined with the carrier'of 105 lnlocyclessuppliedfrorn theintermedia e '7 frequency oscillator 2a.:1rne output o f theintermediate frequency detector thenfrepresents' the original voice frequency spectrumwhichexisted in micr0phon'e'Mof'Ffigure1;i It is obvious that 3, variation in the frequency 0f the incoming 'signal will cause frequency variations to: exist in the [intermediate frequency detector between the 105 75 kilocycle intermediate frequency oscillations and 7 the sideband' frequencies from the bandvpassl,

filter 24, 'In order to eliminatethis difficulty, the

frequencyof the oscillations produced by 2.! is, regulated by a carrier controlled circuit cone nected between the intermediate frequency amplifier 23 and band-pass filter 24. Thiscircuit consists of a 100 kilocycle crystal filter ,29 followed by a phase detector 3|, which-in turn operates i on frequency determining means in the 1 high frequency-oscillator 21 to produce a constant carrier frequency and consequently an intermediate 7 frequency exactly equalto 100 kilocycles,

The crystal filter circuit may be of any type but is preferably of the type disclosed in United" 'States application Serial No. 564,770, filed September 24,1931, Patent No, 2,001,387 and United States application Serial No. 203,901, filed July 7, t

1927, Patent No. 2,005,083, or in UnitedStates application No. 616,803, filed June 13, 1932, Patent No. 2,065,565 or any other type of filter which has the. desired characteristics. The phase detector and oscillator'control means may beof Preferably, this control means 'may 7 any type. be of the type disclosed in United States application Serial No.616,803, filed June 13, 1932, Patent No. 2,065,565. frequencies are, held in proper relation to the oscillator ,28 independently of small variations in the received signal frequency. r 7 In mydescription'of this 'inventionl have so .30' ffar described a transmitter which is'amplitude 'r'nodulated and 'radiatesfa carrier and one side-T 1 V band; I havealso described a receiver designed a V to, receive the modulations of thetransmitter shownin Figure l. n I a Referring to Figure 1, it will be noted that the output of modulator consists of a carrier and,

two sidebands. If the bandepass filter of stage! had been adjusted'to'cover a'frequency band of 190t0 210 kilocycles, bothsidebands would have been passed ontothe modulatorfifl, The lower H sideband then would have consisted of a band of frequencies between 10,190 and 10,1 95 kilo cycles,ia carrier of 10,200 kilocycles and an upper sideband of 10,205 to 10,210 kilocycles.

Th'en' if thefband-pass filter in stage, 9 had beenl adjusted to select, all frequencies betw'een1'0fl90 and 10,210 kilocycles,and'this'energy Z a a had been amplified by the, power lampli'fierj in r stage II), theresulting radiation would have con sistedof' two; sidebands symmetrically spaced about a carrier of 10,200 kilocycles as shown at 'justabove the radio the left of Figure 3, frequency amplifier 2 I.

The receiveriof Figure3 operates in thesame manner as the receiver of Figure 2' with the ex ceptionthat the output of the intermediate fre :7

quency amplifier'23 is associated with two band- I pass filters 3! and38, the first of whichv-r eceivesz' the lower sidebandof '90 to lkilo'cycles and the n p second of which receives the upper sideband of i q '105 to;110, kilocycles. Part-Ofthe kilocy cle carrier output fromthecrystal filter 2 9 isc'orhbined' with oscillations, from a15-kilocycle oscillatori'32in modulator 33', producing two sidebands V of 95 and ikilocycles.,i The lower sideband of 95 kilocycles is selected by, band-pass filter- 43,

mediate frequency detector 39 the, 95"kilocycle carrier is combined with thelower sideband out 'put' from filter3T.) In, intermediate frequency detector 4! the 105 kilocyclecarrier from filter p44 is combined with the upper sideband from 7 71 37 3- p r O tector 39, aswell as while the upper ismeband' r 105: kilocyclesis selected byband-pass filter-M.v In the inter-{1 if 25 this manner'the sidebandji the output of detector 4| now each includes the original voice frequency which impinged on microphone M of Figure 1. Under some conditions, it may be advantageous to combine the output of the upper sideband with the, output from the lower sideband, as just shownin connection with Figure 3, thereby obtaining some improvement due to frequency diversity between the upper and lower sidebands. During other conditions, in which the phases between the upper and lower sidebands are rapidly changing, it would probably be desirable to receive the output produced by one sideband only, as shown in connection with Figure 2.

So far I have described the ,use of my invention for amplitude modulation only. I now propose to briefly describe how the same arrangement may be applied to either phase or frequency modulation.

In Figure 1, if we take the output of the filter in stage 5, we can phase modulate or frequency modulate modulator 30 in the conventional manner. In the case of frequency modulation, it will be noted that the modulation frequencies he between 5 kilocycles and 10 kilocycles, a ratio of 2 to 1. On the other hand, when we frequency modulate a transmitter, in a manner known in the art, with voice frequencies ranging between, say, 50 cycles and 5,000 cycles, we have a ratio of 100 to 1 between the lowest and highest modulation frequencies.

In frequency modulation, the frequency swing is the same for all frequencies which produces an effect equivalent to over-modulation on the low frequencies. In a frequency modulated sideband, the relative amplitudes of the sideband frequencies are inversely proportional to the modulation frequencies, for moderate amounts of modulation, especially at the higher modulation frequencies. That is, the sideband amplitude for a given frequency will be double that for twice that frequency. Thus, if the frequency band extends from 5,000 to 10,000 cycles, the sideband amplitude at 5,000 cycles will be double that at 10,000 cycles for equal input to the modulator at the transmitter. By limiting the frequency range to 2 to 1, as I have done in my invention, it will be noted that the effective percentage of modulation is nearly the same over the entire range of modulation frequencies. The overall result, therefore, is intermediate between normal frequency modulation and phase modulation produced by conventional methods. Furthermore, if we wished to correct frequency modulation to phase modulation we may do so by making the modulation voltage proportional to the modulating frequency, as disclosed in United States application Serial No. 608,383, filed April 30, 1932. Briefly, this may beaccomplished by passing the modulating potentials through a circuit as shown in Fig. 4, including a high resistance R and inductance I in series, and utilizing the modulating potentials from the inductance so that the higher signal frequencies are stressed and the output potentials varied proportionally as the frequency of the input potentials vary. Any other correcting circuit having the characteristics outlined above may be used. The correcting circuit may be used at any point in the audio frequency path at the transmitter, for example, said circuit may be interposed between the speech amplifier I and the modulator 2. The correcting circuit may also be used in any of the audio frequency circuits at the receiver. Of course, the problem of producing phase modulation in this manner is extremely simplified by the present invention because the range of modulating frequencies is limited to only two to one.

The correcting network in the audio frequency output at the receiver may also consist simply of a resistance R and inductance I in series as shown in Fig. 4, with the audio amplifier connected across the inductance; If the series resistance is high enough, the audio frequency current through theinductance will be-substantially independent of the frequency. Consequently, since for the same current, the reactance drop will be twice-as high at 10,000 cycles as at 5,000 cycles, the output at the higher frequencies will be increased in just the right proportion to compensate for the amplitudes as received from the detector.

As a further modification, it is also obvious that the output from the filter in stage 5 of Figure 1 could be used to modulate a high frequency transmitter directly by conventional methods without passing through the additional modulator 40. For example, suppose we should modulate by conventional methods a carrier of 10,200 kilocycles with the output from the filter in stage 5. We would obtain a carrier of 10,200 kilocycles, an upper sideband ranging between 10,205 and 10,210 kilocycles and a lower sideband ranging between 10,190 and 10,195 kilocycles. The modulation could be either amplitude, phase or frequency. The receiver of Figure 3 could be used for receiving such transmissions in exactly the manner described previously in this disclosure.

Since the sidebands are selected and detected separately, no change in adjustment at the receiver would be required when changing from amplitude modulation to phase modulation, providing only one sideband is being received. If both sidebands are being received and combined in the manner shown in Figure 3, it would be necessary to reverse the output of one of the sideband detectors when changing'from amplitude to phase modulation or vice versa, as shown in United States application Serial No. 613,154, filed June 20, 1932.

Having thus disclosed my invention and the operation thereof, what I claim is: i

l. A receiver for receiving signal energy com prising a carrier and two sidebands which during transmission have been separated from said carrier by substantial frequency bands comprising, signal absorbing means, heterodyne demodulating means connected therewith, said demodulating means including a high frequency oscillator, a pair of band-pass filters connected with said demodulating means, each band-pass filter being adapted to pass one sideband only of the de modulated energy, a local oscillator, a modulator coupled thereto to be energized thereby, a filter coupling said modulator to said demodulator to impress filtered energy from said demodulator on said modulator, signal detecting means connected with each of said band-pass filters, separate filters tuned to different frequencies connecting said detecting means to said modulator to impress selected energy from said modulator on said detecting means, a single indicating means connected with said detecting means, and a frequency control device interposed between said high frequency oscillator and the output of said demodulating means to control the frequency of the high frequency oscillator in accordance with energy from said demodulating means.

2. The method of receiving signal energy comprising a carrier and sidebands which during transmission are" spaced by a substantial free quency band whichjincludes' the steps of, combining said rcarrier an'd sidebands with high fre- "'quency oscillations to produce a beatnote'ineluding two signal carrying sidebands, separating saidsidebands; modulating said heat note with a lower frequency to obtain 'two new carrier freproduce indications characteristic oof .thesignal.

' 3. A'signaling system comprising, a source of signal potentials of a pre-determined' frequency. i range, a circuit Vfor'inipressingsaidi'signal p'o-i tentials on carrier frequency oscillations of a value such that 'a side band resulting from said:

modulation extends substantially from the c'ar Vrier'to the'lowes't side band frequency and to the highest side, band-frequency, a; filter connected Withpth output of said modulator for selecting a value that the 'sidegbandsin the output of {said circuito are separated from the carrier 'by a frequency band relati elyJargeJas compared to 'fthe' frequency range o'f itheiside bands whereby lfrcniflthe'othersideband and came; a transmita ene of said side bands may be readily separated Q ting deyicaa filter connecting theoutput of said modulator} to said transmitting device, said filter bein'gfltuned to pass thecarrier one sideband; 1 receiving means tuned to ja'irequencyinterme cliate the carrier frequency and the side'bfand" passed by'said filter; a demodulating circuit including a source'of carrierfrequency'oscillations coupled to" said receiving means; said sourcejof frequency range andfm ay bej lfea'dily separated from the energy resultant from 'saiddemojdulag L4. The-meth d of-receiyings I V prising'a carrier and both'rside'bands;whieh"dur- I; tion',a filter circuit connected with saiddemodu-i lator and signal j translating means connected with said filter;

ing transmissionare spaced from the carrier' by a "substantial frequency bandjwhich includes the V stepsof, combining ,sai'd carrier and side'banjds with high frequency ps'c'illations to produce re sultant 'energy comprising two bands of signal? earrylng'frequencies each materially spaced from '7 new carrier wave, separating said b'andsof signal [carrying frequencies: from each other and from saidnew' carrier wave, rreducing1the freje y Obtained 'limitsi 2 1 V V V 5; A receiyer'for receivingsignal energy 'com- .jqnenc of said bandsto derive 'the'signal ,fsep

arating said new carrier wave from said resultant 7 energy; filtering 'said'sepafrated carrier wave to' remove all fsignal components therefrom; and

futilizing said filteredcar rier wave to contr'ol the frequency or the oscillations combined with said 5' received carrier tomaintainj thelres'ulta'ntreno thereby within predetermined en l energygom- I 2,095,05 1 i V 7 V g V M prising-a carrier and twid side bands which' du'r V i ing transmission have been separated from said carrier by substantial frequency band's compris-i 7 ing signal absorbing means, heterodyne' demodux lating means connected therewith, s'aid' demodu' I lating'i'neans including a high frequency oscil- 7 later, aninterme'diate frequency amplifier ;con-

nected tof'said deni'odulatorg'a pair of ba nd pas's each of said band pass filters, a single indicating I means connected with said detectingmeans, and

akfrequency flcontrol device including fahighly V filters con'nect ed with said intermediate frequen-' 'cyf'amplifief; each band-pass filterbeing adapted topass oneside band only of the demodulated. energy, signalfdeteeting means connected with j lib ' selective filter interposed between said source o 6. The method of signaling which includes'the steps of,distorting'modulating'potentials to relamodulatingpotentials on wave energy of an'intermediate frequency, impressing ,a' sideband, of,

theresultant'energy onlother waveener y ofinl- 20 V tively increase the 'amplitudeof the higher mod- 'ulationr frequencies, impressing *s'aid distorted j one of said side band's, a circuit including a see-.

' 0nd modulator for impressing said selected side band on to carrier frequency oscillations'of sucli' 130i or .7. The method or signaling which includes the:

steps of, impressing 'modulati'ng potentials on waveenergy of anintermediate frequencyto pro- 5 duce a'carrier andsidebands spacedfromsaid carrier'byfrequency bands of'a width substan V selecting one-of said sideband frequenciesfand modulating the frequency of wave energy of high! frequency in accordance with the energy of said 7 selected sidebandto produce-resultantfrequency modulated energy,,,the'effective percentage of 'modulation Of WhiCh'iSl substantially the same p; over-the;entire modulation frequency range; '1

. 8,,The method of-receiving oscillatory energy 1 7 comprising a carriergand 'sidebands the length of which arejmodulated in accordance with signal-{ ing potentials; which sidebands are spacedffroni 7 i V :thecarrierby a frequencybandat least las'wide V V asfthellfrequency band of the 'signalimodulationsi V "which includes the steps of, impressing said ear rier and sidebands .on high frequ'ency oscillations V tofproduce resultant signal carrying energy of U low frequency; impressing saidj resultant signal energy of lowif requency on oscillatoryenergy 70f t w intermediate frequency to render the signal, dis torting the amplitude'of the rendered signal proportionally to its frequency and producing indi- 

